1. Technical Field of the Invention
The present invention relates to photonic detection and more particularly to a photo detector detecting multiple wavelengths simultaneously and connecting to a single substrate.
2. Background Art
It is often desirable in infrared and other visioning systems to be able to detect and determine, on a simultaneous and pixel-registered basis, the amount of light of two or more different wavelengths in a given field of vision. Being able to simultaneously distinguish between these wavelengths of light and to determine the relative amounts of each with a single vision system is important for such purposes as identifying a spectral signature for a given source.
Single color semiconductor detectors and detector arrays are well known in the art, such as HgCdTe type focal plane array detectors. More recently, two color pixel detectors have been introduced. Suppliers such as DRS, Raytheon, Rockwell, and Lockheed Martin will be familiar to those skilled in the art.
The following patents may provide useful context for the description that follows:
Willner et al's U.S. Pat. No. 5,546,209, entitled, “One-To-Many Simultaneous and Reconfigureable Optical Two-Dimensional Plane Interconnections Using Multiple Wavelength, Vertical Cavity, Surface-Emitting Lasers and Wavelength-Dependent Detector Planes, issued Aug. 13, 1996, involves the use of multiple semiconductor photodetector devices comprised of interband absorption materials and with different absorption spectra. These devices are integrated onto separate, optically transparent substrates, and then stacked one on top of the other to achieve multi-wavelength absorption in a pixel-registered fashion. The device uses wavelength-division-multiplexing (WDM) to facilitate simultaneous and reconfigurable communication of from one, to many, 2-D optical planes.
Schimert's U.S. Pat. No. 5,539,206, entitled “Enhanced Quantum Well Infrared Photodetector,” issued Jul. 23, 1996, discloses an infrared detector array that includes a plurality of detector pixel structures, each of which has a plurality of elongate quantum well infrared radiation absorbing photoconductor (QWIP) elements. The group of QWIP elements are spaced such that they comprise a diffraction grating for the received infrared radiation. An infrared radiation reflector is provided to form an optical cavity for receiving infrared radiation. A plurality of detector pixel structures are combined to form a focal plane array. Each detector pixel structure produces a signal that is transmitted to a read out circuit. The group of the signals from the detector pixel structures produces an image corresponding to the received infrared radiation.
Choi's U.S. Pat. No. 5,384,469, entitled “Voltage-Tunable, Multicolor Infrared Detectors,” issued Jan. 24, 1995, discloses a tunable radiation detector consisting of a superlattice structure with a plurality of quantum well units, each separated by a first potential barrier and each having at least two doped quantum wells separated by a second potential barrier. The wells each have a lower energy level and a higher energy level. The first potential barriers substantially impede penetration of electrons at the lower levels. The second potential barriers permit electrons at the lower levels to tunnel through, and prevent energy-level coupling between adjacent doped quantum wells. A biasing circuit is connected across the semiconductor superlattice structure. A photocurrent sensor is provided for measuring the amount of radiation absorbed by the semiconductor superlattice structure. The superlattice structure is made a part of a hot-electron transistor for providing amplification.
Dreiske's U.S. Pat. No. 5,818,051, issued Oct. 6, 1998, for a Multiple Color Infrared Detector, discloses a detector formed from a photodiode, a photoconductor, and an insulating layer of material disposed between the photodiode and the photoconductor. The photodiode detects infrared radiation in the spectral band between about 3 and 5 micrometers, and the photoconductor detects infrared radiation in the spectral band between about 8 and 13 micrometers.
Chapman's U.S. Pat. No. 5,959,339, issued Sep. 28, 1999, entitled Simultaneous Two-Wavelength P-N-P-N Infrared Detector, discloses a detector array architecture with two stacked detectors of different wavelengths per pixel, where a pair of contact points for reading the current in each of the two detectors is brought by a metalization layer to the back surface of the device for pixel discrete contact with the ROIC substrate. The current induced by the first wavelength is measured directly at the first detector contact, while the current induced by the second wavelength is calculable in ROIC as the difference between the currents measured at the respective first and second detector contacts. There is a common edge conductor applied to the deepest contact on the pixel, the contact layer closest to the face of the detector, for supplying the voltage bias for the two detectors. The conductor connects all pixels and extends to the edge of the array where it is connected to the voltage source.
The Chapman disclosure is noteworthy in its description of the construct of its FIG. 2 embodiment device; the transparent substrate being first coated with an N layer of about 11 microns, a first P layer of about 3.5 microns, an N layer of about 8.5 microns, then a capping P layer of “less than” 3.5 microns. The configuration of the backside contacts includes an intentional short between the first P layer and 2nd N layer, to avoid “an undesirable additional indium bump [or discrete pixel connection to the ROIC, which] would complicate the fabrication of the detector, and may also increase the area of the unit cell.” The larger implication in this statement is representative of the problem with this type of architecture; the thickness of the device when the prior art methodology is extended to the multiple layers necessary to construct a multiple wavelength detector, is not conducive to the addition of any further backside contacts due to the depth of the wells or vias.
In summary, there is not demonstrated or anticipated in the art a multi-color, multi-focal plane optical detector in a monolithic or unitary device that can be fabricated by deposition techniques on a single substrate with a sufficient number of back side contacts suitable for direct, discrete, pixel to ROIC connection and direct current readout for each wavelength, and that can be easily scaled up to large array configurations suitable for the many applications for which such a device would be attractive.